Communication breakthrough. Researchers have developed a structured material that permits sound waves to pass more easily from water to air or vice versa.×

Sound waves mostly reflect back from any water-air boundary, making it nearly impossible to hear underwater sounds from above. But now physicists have devised a structure that, when placed in contact with the surface, can enhance sound transmission up to 160 times, allowing 30% of the sound energy through. The technology could be used to help communications with people underwater or in monitoring ocean environments. With further development, the sound-transmitting materials could lead to more sensitive underwater sound detection.

Sound is partially reflected whenever it encounters a boundary between two substances of different density and sound velocity. It’s possible to reduce the reflection—and correspondingly increase transmission—by placing a material with intermediate sound-response properties at the boundary [1]. One such acoustic coupling material is the gel used during a medical ultrasound scan to help sound waves pass from the body into the detector. However, water and air are so different that no common substance has the right intermediate properties.

Researchers led by Sam Lee of Yonsei University in Seoul, South Korea, have now demonstrated an alternative technique: enhance transmission by using a metamaterial—a structure designed to exhibit properties unlike any natural substance. In this case, the structure is a cylindrical shell, or cavity, with a thin, plastic membrane at one end. The membrane is divided into segments by a rigid frame, and a pill-shaped mass is attached to the central segment. When placed at the air-water boundary and hit by an incident sound wave, the structure responds by generating a secondary wave. The team could tailor this secondary wave by adjusting the length of the cavity, the tension of the membrane, and the size of the mass. Based on calculations, they chose a structure whose secondary wave interferes destructively with wave reflection and thereby enhances transmission.

To test this structure, Lee and colleagues experimented with sound propagating through a vertical tube of diameter 30 mm, separated into two compartments by a thin plastic separator. The upper compartment was filled with air, while the lower one contained water. Before adding the metamaterial structure, the team measured the transmission of sound waves from the water side to the air side using waves with frequencies from 600 to 800 Hz. They found that, at all frequencies, only about 0.2% of the wave energy came through.

But with their metamaterial structure in contact with the plastic separator, the boundary let as much as 30% of the wave energy through at frequencies around 700 Hz. For other frequencies in the range from 650 to 750 Hz, transmission was also enhanced, though not as strongly. The team stresses that the frequency of maximum transmission can be altered—or even extended to a wide range of frequencies—by changing the membrane tension or by making other adjustments to the metamaterial structure.

The team performed detailed numerical simulations that reproduced the observed wave behavior. They also simulated placing several of their ring structures next to each other, with no overlap, to create an array that could enhance transmission over an extended surface.

Lee and colleagues expect that such metamaterials should find many uses in helping sound travel either from water into air or vice versa. For example, the technology could improve underwater microphones (hydrophones), which are currently about 1000 times less sensitive than microphones that operate in air. One could imagine a hybrid device in which an air microphone is in an airtight cavity surrounded by a metamaterial interface. Placed underwater, the metamaterial would transmit sound waves into the cavity, where they could be picked up by the microphone. The interface, the researchers suggest, would allow underwater sound detection with 10 times the sensitivity of the current best hydrophones.

Metamaterials physicist Ping Sheng of the Hong Kong University of Science and Technology thinks the authors have found an extremely simple solution to the problem of sound transmission across a boundary. “The thing that is surprising is why this was not demonstrated a long time ago,” he says. Daniel Torrent, also a specialist in metamaterials, from the University of Bordeaux in France, thinks this technology could offer a noncontact way to do ultrasound imaging, as the metamaterial could enhance transmission of sound waves from a water-based object of study through the air to a detector.